Gangliosides appear to be important recognition molecules on the cell surface. 1. The membrane receptor for cholera toxin is believed to be the ganglioside GM1. We have directly demonstrated that the receptor for cholera toxin on rat intestinal brush borders is GM1 and glycoproteins. 2. Previous studies suggested that gangliosides may be involved in the binding and action of thyrotropin to thyroid cells. Using a clonal line of normal rat thyroid cells, we have been able to demonstrate that gangliosides do not play a role in thyrotropin binding or action. 3. Gangliosides inhibit the attachment of cells to collagen which is mediated by fibronectin, a cell surface glycoprotein. Cells that are deficient in gangliosides synthesize but do not retain fibronectin. When the cells are treated with gangliosides, the cells retain fibronectin and organize it into fibrillar arrays. 4. Cultured cells incorporate radioactive precursors into their gangliosides within 5 min, but the labeled gangliosides do not appear on the cell surface until after 30 min. This lag appears to represent the time required for the newly synthesized gangliosides to be translocated from the Golgi apparatus to the plasma membrane.